The letter tells the reservists which regiment or unit to report to in the event of hostilities. “Attached you will find your personal details as well as your role in the event of war,” it reads.

One Finnish reservist, who received the letter, said: “The timing was not random. It is clearly due to a more aggressive stance by the Russians. I’ve been in the reserves for 15 years and this is the first time I’ve received something like this. They send out letters like this very rarely.”

Finland’s army has 16,000 soldiers, but it could expand to 285,000 if reserves were to be called up.

“The reservist letter is associated with our intention to develop communications with our reservists, and not the prevailing security situation,” said Mika Kalliomaa, a spokesman for the Finnish Defence Forces.

The aim was to check that the armed forces had the right contact details for all reservists, he added.

But experts said that even if the initiative pre-dated Russia’s seizure of Crimea, the letter was clearly prompted by worries about the Kremlin’s intentions.

“If Russia had headed down the path towards being a liberal democracy, there would not have been the pressure to do this,” said Charly Salonius-Pasternak, a senior research fellow at the Finnish Institute of International Affairs.

“In the current reality, it makes sense. The Finnish Defence Forces want to make sure that if they need to blow the whistle, they can rely on 230,000 reserves.”

Mr Salonius-Pasternak added: “That is linked to the increasing instability in the region. Russia has shown that it can transport large numbers of troops across vast distances very quickly. I have never had so many people coming up to me asking if they should be worried about the security situation.”

The Soviet Union invaded Finland in 1939 and seized more than 10 per cent of the country’s territory. During the Cold War, Finland was officially neutral, but remained under the influence of its neighbour.

Neighbouring countries are also on a heightened state of alert. Last October, Sweden carried out its biggest military mobilisation since the Cold War to hunt for a mysterious submarine sighted near Stockholm.

Although not a member of Nato, Finland has strengthened its ties with the Atlantic Alliance. Last month, the country promised more military cooperation with the armed forces of other Nordic countries.

On Thursday, David Cameron joined EU leaders for a summit with six former Soviet states in Riga, the Latvian capital.